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Full text of "Handbook Of Chemical Engineering - I"

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EVAPORATION                                         369
ent upon the available temperature differences between the heating steam and the vacuum and this temperature difference is frequently influenced by the increased boiling point of the solution (caustic soda, calcium chloride).
Evaporators are usually operated with exhaust steam at from 0 to 10 Ib. pressure, and with live steam up to 30 Ib. pressure, as a higher pressure will greatly increase the cost of the apparatus. The vacuum in the last effect depends on the amount and temperature of condensing water available, and for all smaller installations and ordinary liquors, a wet system condenser with from 25 to 26 in. of vacuum is the usual practice. For larger plants and for delicate liquors, it is important to have a high vacuum, which must be produced in a barometric condenser with dry-vacuum pump, or in a surface condenser in cases where it is not permissible to mix the vapors with the cooling water. Usually a vacuum of from 27^ to 28 in. is carried in these systems, and it is very seldom that a higher vacuum than 28 in. is required.
It has been shown under "Heat Transmission" that the temperature differences in a multiple effect are not evenly distributed for the reason that the factor of heat transmission increases and decreases with the steam pressure. In the earlier stages of evaporator manufacture, multiple effects have been built with various sizes of heating surface so that the temperature difference between each effect would be the same, claiming that this would give better efficiency and capacity. All modern evaporators are built with equal heating surface in each effect, as this will naturally reduce the cost of production, and it has been demonstrated under these conditions each effect will do its proper share of work. Only in cases where large amounts of extra steam are to be taken from one or more effects of a multiple effect, it has been found necessary to increase the heating surface in proportion to the extra amount of vapor to be produced. Such evaporators are frequently installed in beet-sugar factories where large amounts of extra steam are used for the preheating of the weak juice.
Material of Construction.—A good quality of close-grained cast iron should be used for all evaporator bodies as it will reduce the amount of leakage to a minimum. In all plate rnetal and copper construction, the large amount of rivets and seams are always a source of trouble. Recently steel-plate evaporators have been used with welded seams, but this method of construction has not been satisfactory for strong alkaline solutions. The heating surface mostly consists of tubes made of steel, wrought iron, charcoal iron, copper, brass, bronze, aluminum and lead. Evaporators have been built with aluminum shells and of glass enameled steel. There is practically no limit as to what material can be used for the construction of evaporators, and in some cases, steel and cast-iron shells have been lined with acid-proof brick in order to reduce the action of acid liquors on the metal shell. Evaporators have been built of solid lead with lead coils, and also of cast iron with homogeneous lead coating, and sometimes a separate lead lining. Special information on this subject will be given in later paragraphs.
Jacket and Coil Type.—Figure 5 shows the construction of the first multiple-effect evaporator invented and patented by Pecqueur, and the heating surface consists simply of dished copper bottoms. The whole arrangement of the multiple effect is very simple, but naturally the capacity is small as such a construction is always limited in size. Figure 6 shows Howard's vacuum pan with jacket for the concentration of sugar juice. A similar pan was used by Roth for the same purpose.